Field
The present disclosure is directed to an edge lighted backlight unit for a liquid crystal display device, and more particularly a backlight unit comprising a light coupler configured to couple light between a light source and a light guide plate comprising the backlight unit.
Technical Background
Liquid crystal display (LCD) panels typically comprise two pieces of thin glass (color filter substrate and TFT substrate) including a liquid crystal material disposed therebetween, a polymer light guide (e.g. Poly(methyl methacrylate)—PMMA) and a few thin plastic films (diffusers, DBEF films, etc.). Due to the poor elastic modulus of PMMA, the overall structure may have insufficient rigidity to withstand significant physical shock, particularly for large display panels. Young's modulus for PMMA is about 2 gigaPascal (GPa). In contrast, most silica-based glasses have a young's modulus of about 72 GPa. Consequently, additional mechanical structure may be needed to provide stiffness.
Humidity testing shows that PMMA is sensitive to moisture, and dimensions can change by up to about 0.5%. For a polymer light guide with a length of about a meter, this means about 5 millimeters of dimension increase, which is significant and thus makes mechanical design of a backlight unit challenging. Typically, an air gap is provided between the light source (e.g. light emitting diodes—LEDs) and the PMMA light guide to accommodate expansion of the PMMA. Unfortunately, light coupling between the light source and the PMMA light guide is sensitive to the distance between the light source and the light guide, which can cause the display brightness to change as a function of humidity.
PMMA has a coefficient of thermal expansion (CTE) of approximately 75×10−6/° C. and has a relatively low thermal conductivity of about 0.2 W/m/K, while the same attributes for a silica-based glass may be 8×10−6/° C. and 0.8 W/m/K. PMMA also has a plastic transition temperature of about 105° C. The low thermal conductivity of PMMA impedes heat dissipation from the material. Thus, as the light guide approaches close to the light source, which can dissipate a lot of energy, the PMMA can become very hot.
Due to the poor thermal, mechanical and dimensional stability of PMMA, a glass, such as a silica-based glass, is considered a potential replacement solution for PMMA in liquid crystal display (LCD) backlight units. However, since high quality optical-grade glass can be more expensive to produce than PMMA, the cost of the glass drives technology development toward thinner light guides, while the size of the LED's that must couple to the light guide, for example at least 2 millimeters thick in some cases, require the glass to be thicker than traditional 0.07 millimeter (mm) thick LCD glass to enable close to 100% coupling of light from the LEDs into the light guide.